1. Field of the Invention
The present invention relates to a hydrogen-permeable structure including a metal base having hydrogen permeability (hereinafter, referred to as “hydrogen-permeable base”) and an oxide proton conductive film, a manufacturing method thereof, and a fuel cell using the hydrogen-permeable structure.
2. Description of the Background Art
A hydrogen-permeable structure in which a proton-conductive solid electrolyte film is formed on a hydrogen-permeable base has a function of selectively detecting and separating hydrogen and a function of outputting electric energy, and is used in a hydrogen sensor, a fuel cell (hydrogen fuel cell), and the like.
Such a hydrogen-permeable structure is described, for example, in SOLID STATE IONICS, 162-163 (2003), pp. 291-296, and in this document, palladium (Pd) or palladium-containing metal (Pd alloy) is disclosed as a material of the hydrogen-permeable base. As a material of the proton-conductive solid electrolyte film, oxides containing alkaline earth metal and cerium (Ce) or the like are disclosed.
Here, as an oxide that forms a solid electrolyte film, a composite oxide having a basic composition represented by a general formula of ALO3 (in which A represents alkaline earth metal and L represents tetravalent element such as Ce, titanium (Ti), zirconium (Zr) and the like) and having a perovskite-type crystal structure (hereinafter, referred to as “perovskite structure oxide”) can be preferably recited because it has excellent heat resistance. Among these, a variety of perovskite structure oxides having such a composition that tetravalent elements L are partially substituted by trivalent elements M are proposed because they have excellent proton conductivity.
Also it is demanded to reduce film thickness of a solid electrolyte film in order to realize excellent proton conductivity. Such a hydrogen-permeable structure may be obtained by forming a proton conductive thin film on a hydrogen-permeable base made of, for example, Pd or Pd alloy, by vapor-deposition of material elements constituting the perovskite structure oxide by sputtering method, laser ablation method (pulse laser deposition method or PLD method) and the like.
In forming a solid electrolyte film, namely, a proton conductive thin film (hereinafter, referred to as “proton conductive film”) by the method as described above, if the surface of the hydrogen-permeable base has roughness, defects are likely to occur in the proton conductive film. Therefore it is demanded to remove roughness occurring on the surface of the hydrogen-permeable base. In particular, when the hydrogen-permeable base is Pd foil or Pd alloy foil, rolling, surface grinding or the like for removal of roughness is conducted because such foil is soft and roughness is more like to occur on the surface thereof.